This invention relates generally to apparatus for use in conjunction with injection molding machines for rapidly cooling work pieces molded by the molding machine. The invention relates particularly to a method and apparatus for rapidly cooling injection molding parisons to be employed in the manufacture of biaxially oriented articles such as containers for carbonated beverages and the like.
The manufacture of biaxially oriented containers such as those formed polyethylene terephthalate (PET), employs an initial step of forming a parison or preform by an injection molding process. The parisons are generally in the shape of a hollow cylinder closed at one end and are often formed in multicavity molds into which plastic material is injected around a core positioned within each cavity. Generally the core and cavity are cooled by means of a liquid flowing through channels within the walls of the core and cavity thereby cooling the plastic forming the parison to a form-stable shape defined by the core and cavity.
Once the parison is formed and cooling is initiated, the core and parison are removed from the cavity thereby exposing the outer surface of the parison to room temperature air. An ejector or stripper mechanism then strips the parisons from the cores to expose the inside surface of the parisons to room temperature air and to free the molding machine to initiate another molding cycle. As the parisons are stripped from the cores, any contribution by the cores to the form stability of the parisons is also removed thereby freeing the parisons to be modified from their initially molded shape by outside forces.
In order to reduce this post molding modification of parison shape, it is desirable that the preform be cooled as much as possible thereby enhancing its form stability. The cooling can be enhanced by merely retaining the molded article in contact with the cooled cavity and core for an extended period of time thereby allowing additional heat to be extracted from the parison. This has the undesirable affect of extending the time for each injection molding cycle thereby reducing the number of parisons which can be produced in a given unit of time.
In order to enhance the cooling of the parisons, it has been suggested to provide additional cooling apparatus. In one such apparatus disclosed in U.S. Pat. No. 4,472,131, the parison, while retained on the core rod of the mold, is positioned within a supercooling mold. The outer surface of the parison cools either by direct contact with the wall of the supercooling mold or by cold air passing through an air gap between the outer surface of the parison and the supercooling mold. The cold compressed air is introduced at a point corresponding to the tip or sprue end of the preform. After passing along the length of the outer wall of the preform, the air escapes through holes located in the mold adjacent to the preform neck ring or thread finish During the cooling, the parison is retained on the core rod of the initial mold in which the parison was formed. Thus in order that the molding cycle of the molding machine may continue, duplicate core rods and shifting mechanisms for those core rods must be supplied to permit a reasonable production of parisons in a given unit of time.
In another disclosure of cooling apparatus found in U.S. Pat. No. 4,592,719, the parison is removed from the core rod on which it was formed by a pneumatic grip which draws atmospheric air by suction along the length of the parison between the parison outer wall and the pneumatic grip. The grip then removes the parison from the molding machine where a second grip is introduced into the interior of the parison. The second grip also causes atmospheric air to be sucked along the inner walls of the parison thereby enhancing the cooling of the parison through turbulent contact with a large volume of ambient air.
While such apparatus speeds the cooling of the parisons to an acceptable form stable temperature in a shorter time than would be experienced by mere contact with ambient air in the absence of such apparatus, further reductions in cooling time are desirable which would allow even additional increases in parison production from a given molding machine. Thus an object of the present invention is to develop a method and apparatus for reducing the time period of the cooling of the preform subsequent to injection molding thereby enhancing the productivity of the associated injection molding machine.